Sensorineural hearing loss results from damage to or deterioration of the inner ear (sensory loss) and/or damage to the auditory nerve (neural loss) which conducts the signals to the brain. Sensorineural hearing loss can be caused by diseases, birth injury, drugs that are toxic to the auditory system, and genetic syndromes. Sensorineural hearing loss may also occur as a result of noise exposure, viruses, head trauma, aging, and tumors. Except in the case of a sudden onset of hearing loss due to certain injuries, sensorineural hearing loss is generally not reversible.
It has been estimated that 80% of all cases of hearing loss are sensorineural. In the United States alone, it is believed that more than twenty two million people have enough sensorineural hearing loss to affect their ability to communicate. This form of hearing loss is particularly devastating because it affects the ability to discriminate between sounds and understand complex combinations of sounds, including speech.
Generally, sensorineural hearing loss is a cumulative disease caused by a wide range of genetic and environmental factors. The effects of these factors include degradation of the myelin sheaths enclosing auditory nerves, neurological plaque formation causing cell/neuron death/dysfunction, and cochlear receptor cell death. An individual's genetics determine the threshold resistance of the cochlear system and associated neurons to cumulative damage before the effect of environmental and external factors. Predisposition to sensorineural hearing loss may be evidenced by ear-specific genetic variations that may be inherited variations, whether dominant, recessive and/or sex-linked. Some individuals may have a genetic predisposition to spontaneous mutation.
Known genetic variations may have a number of effects contributing to sensorineural hearing loss. These variations may result in improper enzyme or cell physiology including deficient enzyme function in neural myelination, ineffective or malformed membrane transporters for intracellular nutrients such as thiamine, poor native production of native anti-oxidants such as dismutases and synthases. Other effects may include increased production of super-oxides, hydroxyls, and other damaging molecules and radicals, decreased mitochondrial health secondary to genetic coded enzyme misconformation and ineffective metabolic degradation of toxins, evidenced in certain individuals that have a greater sensitivity to a type of antibiotics called aminoglycosides.
Genetic variation may result in poor cell membrane stability, impaired cellular microstructure with poor microcirculation exacerbating all of these effects. Systemic genetic diseases such as diabetes may result in the formation of glycation products leading to respiratory burst enzymes e.g., superoxide anions. Similarly, hypertension may result in upregulation of NAD(P)H oxidase, a source of superoxide anions. Superoxides and similar reactive oxygen species can cause oxidative damage to cell structures. Hydroxyl radicals can damage cell membranes and lipoproteins by lipid peroxidation. Radical oxygen species can damage proteins, leading to structural changes and loss of enzyme activity.
Environmental and other external factors may contribute to cumulative damage to inner and outer hair cells, cochlear mitochondria, membrane stability and the vascular supply to cochlea and nerves. Continuous exposure to noise levels above 85 dB will cause gradual hearing loss in some individuals. Even a single exposure to higher noise levels may cause damage. Certain medications such as aminoglycosides, vancomycin, furosamide, and chemotherapeutic agents can damage the cochlea system. Infections such as meningitis, mumps, measles, other viral and bacterial infections, allergic reactions, exposure to ionizing radiation and autoimmune deficiencies can all contribute to sensorineural hearing loss.
The concept that sensorineural hearing loss may result from the cumulative affect of multiple insults prior to manifesting irreversible damage is well recognized and described. These multiple insults gradually wear down the buffer zone between normal auditory function and the onset of hearing loss. Age is associated with hearing loss because longer life translates into more cumulative exposures to causes of the loss. If there is a family history of hearing loss, e.g. a genetic predisposition to any of the many factors that can result in sensorineural hearing loss, the buffer between normal function and deterioration is smaller than in non-genetic cases.